In an optical fiber used for transmitting data, it is sometimes necessary to cause the refractive index of the optical fiber to vary periodically so as to use it as a reflector, in particular for obtaining a wide-spectrum filter for compensating high chromatic dispersion over a large spectrum band by means of the various spectrum components being reflected in staggered manner. Such index variation is achieved by inscribing a Bragg grating. This is done by irradiating the fiber with a system of interference fringes formed by two secondary ultraviolet beams coming from a common primary beam, and sloping symmetrically. The gap between the fringes defines the pitch of the grating.
There are several prior art solutions for obtaining a variable-pitch Bragg grating. A first solution consists in illuminating, with a parallel primary beam, a "phase grating" formed of parallel lines ruled in a transparent plate. The thicknesses of the lines are chosen so as to eliminate the diffracted beam of order 0. The beams of order +1 and -1 constitute the two secondary beams. Discrete variation in the pitch between successive segments of the phase grating causes corresponding variation in the pitch of the Bragg grating. Unfortunately, it is difficult to make the difference in pitch between two successive segments of the phase grating less than 1 nm. This difference is too great to make a filter having high chromatic dispersion, since it can be necessary in such a filter to obtain a pitch difference of less than 10.sup.-11 m, i.e. 0.1 angstroms.
Another prior art process consists in forming the two secondary beams from a parallel primary beam by means of two mirrors. De-collimating a secondary beam causes longitudinal variation in the pitch of the system of fringes. But the inscribed grating obtained by performing that process is too short, e.g. limiting the spectrum band of a chromatic dispersion compensator.